Magnetic Drag on Hot Jupiter Atmospheric Winds

TL;DR

This paper investigates how magnetic drag influences atmospheric winds on hot Jupiters, showing it can significantly limit wind speeds and affect flow patterns, especially considering geometrical and temporal variations.

Contribution

It provides the first detailed evaluation of magnetic drag effects in a 3D atmospheric model of a hot Jupiter, highlighting its potential to alter wind dynamics.

Findings

Magnetic drag can plausibly limit wind speeds on hot Jupiters.

Magnetic drag exhibits strong geometrical dependence affecting flow patterns.

Transient heating in eccentric planets causes time-variable magnetic drag levels.

Abstract

Hot Jupiters, with atmospheric temperatures T ~ 1000 K, have residual thermal ionization levels sufficient for the interaction of the ions with the planetary magnetic field to result in a sizable magnetic drag on the (neutral) atmospheric winds. We evaluate the magnitude of magnetic drag in a representative three-dimensional atmospheric model of the hot Jupiter HD 209458b and find that it is a plausible mechanism to limit wind speeds in this class of atmospheres. Magnetic drag has a strong geometrical dependence, both meridionally and from the day to the night side (in the upper atmosphere), which could have interesting consequences for the atmospheric flow pattern. By extension, close-in eccentric planets with transiently heated atmospheres will experience time-variable levels of magnetic drag. A robust treatment of magnetic drag in circulation models for hot atmospheres may require iterated solutions to the magnetic induction and Saha equations as the hydrodynamical flow is evolved.